1. Field of the Invention
The present invention relates to electrophotographic apparatus and more particularly to such apparatus having improved corona discharge devices for effecting primary charging of moving photoconductors.
2. Description of the Prior Art
In the field of electrophotography the quality of the final image is affected significantly by the consistency of the primary, i.e., pre-exposure, charging of the photoconductor imaging member. Consistency in this sense includes both the overall uniformity of potential level throughout a particular image area and the constancy of such potential level with respect to each successive image area.
A certain amount of relative movement between the photoconductor surface and the charging unit is helpful towards achieving intra-image uniformity. However, in modern continuous copiers, e.g. of the type producing more than 3000 copies per hour, the problem of providing a constant potential level on successive photoconductor surfaces during the short period in which they rapidly pass the primary charging station is substantial.
For example, in such high speed operation variations in the photoconductor velocity, the photoconductor to discharge electrode spacing and the photoconductor capacitance all are possible causes for non-uniform charging. Also, variations in the efficiency of the charging device, caused, e.g., by change in humidity, barometric pressure or temperature, and by aging of the electrode and fluctuation in line current, present further chance for inconsistency of inter-image potentials.
Open wire DC corona chargers have a rapid charging rate which would be suitable for achieving adequate charge magnitude on such rapidly moving photoconductor at relatively low energizing potentials; however, these devices are highly sensitive to all or most of the system and environmental variables mentioned above.
Grid-controlled DC chargers are fairly insensitive to the variations characterized as the "charger efficiency" type because the level of charge applied by the devices is controlled by the field between the photoconductor surface and their fixed-potential grid. For this reason that technique has become a commercially preferred one for high speed applications. However, the level of energizing voltage required for grid-controlled devices to achieve proper charging at high photoconductor speeds produces a significant quantity of ozone. This aspect can necessitate safety devices and is sometimes damaging to operating parts of the copiers. In addition, grid-controlled chargers usually do not attain an equilibrium photoconductor potential in high speed charging; and the devices therefore continue to suffer a significant sensitivity to variations in photoconductor velocity, capacitance and spacing.
DC-biased AC charging devices present an alternative which is attrative (in comparison to grid-controlled charging) from the viewpoint of lessening ozone. These devices also can provide some degree of charge level regulation because a charging equilibrium is reached when charging current in the positive and negative cycles is equal (see e.g. U.S. Pat. No. 3,076,092). However, as in grid-controlled devices, this control is not complete when operating in high speed devices where charging time is insufficient to reach complete equilibrium. Thus such devices are also sensitive to variations in photoconductor velocity, capacitance and spacing. Further, since the control effect in DC-biased AC charging is based on a balance of charging current, these devices are also sensitive to variations in humidity, barometric pressure, temperature, electrode age and line current.
In view of the various problems connected with each of the different general techniques discussed above, a variety of hybrid or combination approaches have been suggested. For example, U.S. Pat. No. 2,778,946 discloses utilization of an initial open wire DC charger to place up to about 80% of the desired level of charge, followed by a grid-controlled DC charger which provides the remaining 20% required to establish the photoconductor surface at the desired primary charge level. This approach serves to facilitate operation of the grid-control effect closer to a zero photoconductor-grid field condition and therefore decreases the sensitivity of the system to variations in velocity, capacitance and spacing of the photoconductor. However, the system still remains sensitized in some degree to such variations, and the problem of production of ozone is not obviated. U.S. Pat. No. 3,678,350 discloses a similar approach but further provides for the sensing of the charge level intermediate the first and second charging devices and for adjustment of the second charger in accordance with the extent which the initial charge is below the desired level.
U.S. Pat. No. 3,456,109 discloses a different approach. This charging system uses two open wire DC corona chargers, one operative to charge the photoconductor to a saturation level with a first polarity charge and the other providing a subsequent, opposite-polarity charge which "modulates" the first charge and provides charge uniformity within an imaging area. However, it appears that this system remains susceptible to severe inter-image charge level differences created by variations in charging efficiency of the second "modulating" electrode and by variations in speed and spacing of the photoconductor during its movement therepast.